Automated cartridge libraries store and manage large numbers of data cartridges, typically containing magnetic tape on which data is recorded. These libraries are comprised of arrays of storage cells, each cell typically being formed to contain a single data cartridge. The arrays each hold a plurality of data cartridges, and each data cartridge typically has some kind of identifying information, such as a label or bar code. A robot arm, having an optical system for selecting cartridges, is operable to locate a selected storage cell and use an associated reach mechanism to retrieve a data cartridge from the selected cell.
The storage cells are preferably constructed with exact dimensions that correspond to the size of the cartridges, since the cartridges must be positioned in a precise manner if the robotic arm is to grasp them correctly. However, mechanical frame and robot arm tolerances are large enough to cause significant variation in the cell depth within a given library, where cell depth is defined as the distance from the robot arm to the face of a cartridge stored in a particular cell. As a result, it is difficult for the reach mechanism to always extend a specific distance and be assured of reliably engaging a cartridge.
A few prior art systems have addressed the problem of cell depth variation. For example, U.S. Pat. No. 5,040,159 issued to Oliver et al. discloses an optical disk handling system which uses a controller to simultaneously control the position of the reach mechanism and to monitor the force exerted by the motor via shaft encoders and motor current and voltage feedback. The reach mechanism is moved toward a cartridge until a specified force exerted by the motor is encountered, indicating engagement with the cartridge.
As another example, U.S. Pat. No. 5,790,338 issued to Kanai et al. discloses a library apparatus having a photosensor mounted to the tip of the reach mechanism. A robot controller calculates the position of the selected cell and moves the reach mechanism to the calculated position in a coarse control. Then, a target element relating to the selected cell is detected based on the signal from the photosensor. A position recognition controller determines the displacement of the actual position of the selected cell from the calculated position, causing the robot controller to further position the reach mechanism in fine control.
Although the above systems do provide solutions to accommodating variable cell depth within an automated storage library, these solutions are overly complex, costly, and may be susceptible to failure.
Therefore, a need exists for a system and method of adaptive cartridge engagement that reduces the complexity of the control systems and associated software, eliminates the need for multiple sensors mounted on the moving reach mechanism which are subject to dust and flexible cabling failures, and increases the reliability of cartridge engagement by the reach mechanism.